For example, Patent Literatures 1 to 4 disclose technologies for recognizing the state of a human in a specific area, such as inside a room, or the presence of a human in a room by using a human sensor. The technologies disclosed in Patent Literatures 1 and 2 involve, for example, detecting the presence of a human in a room based on body movement or biological information obtained from a sensor. Patent Literature 1 discloses a technology for estimating the state of the room from three states, namely, absent, resting, and active states, by performing a threshold determination process based on the intensity of a signal of a Doppler sensor radiated into the room and a variance value. Patent Literature 2 discloses a technology for extracting a respiration component, a heartbeat component, and a body-movement component by performing frequency conversion and a filtering process on a signal from a pressure sensor attached to a life support device so as to detect the absence or presence from these components or to detect an emergency situation.
Patent Literatures 3 and 4 disclose technologies for differentiating sleeping states and performing problem detection intended for performing monitoring during sleeping. Patent Literature 3 discloses a technology for detecting, for example, respiration, a roll-over, or a fall-off from a bed by radiating a signal from a Doppler sensor toward a human sleeping on the bed and then performing a threshold determination process based on information such as an operating time, speed, and direction obtained from the Doppler signal. Patent Literature 4 discloses a technology in which frequency conversion is performed on a binary signal, which indicates whether or not there is a response from a sensor within a certain short period of time, by using, for example, an infrared human sensor, and it is determined whether a human is in a sleeping state, an awakened state, or an absent state based on a period of the signal.